This invention relates to novel olefin polymer compositions, to a process for making foamed or expanded articles therefrom and to foams of these olefin polymer compositions which are characterized, in fresh or non-fully cured form, by improved elevated temperature distortion characteristics. In addition the foamed products prepared from the present compositions possess improved electrostatic dissipation properties.
It is common practice to make closed-cell ethylenic polymer foams by an extrusion foaming process comprising the steps of heat plastifying a thermoplastic ethylenic polymer: admixing the heat plastified polymer under pressure with a volatile material such as a chlorofluorocarbon and passing the resulting mixture through a die opening or shaping orifice into a zone of lower pressure to vaporize the volatile constituent and form a cellular structure which upon cooling forms the corresponding cellular solid ethylenic polymer foam. However, the development of a viable commercial extrusion foaming process--especially for relatively low density foams of lower olefin polymers--requires balancing a number of different (and sometimes competing) business and technical requirements. Accordingly, much of the extrusion foaming technology is empirical, based upon experience and directed to very specific materials and processes to produce specific commercial products.
One of the requirements for acceptable olefin polymer foam products of growing importance is the elevated temperature distortion characteristics of such products in fresh or partially cured form (i.e., the dimensional stability at elevated temperature of the foam prior to sufficient storage or aging to permit substantially complete replacement of the original blowing agent within the cellular structure of the foam with air). Such characteristic is of growing importance because (a) the elevated temperature distortion characteristics of relatively fresh olefin polymer foams are significantly dependent upon the particular ingredients (e.g., blowing agent, additives, etc.) employed in its preparation: (b) warehousing costs, storage space factors, and/or other considerations oftentimes mandate shipment of the olefin polymer foam products from the manufacturing facility prior to the time that they become fully cured: and (c) such partially cured foam products may, during transport, in enclosed vehicles particularly in warm climates or during hot weather, be subjected to prolonged exposure at relatively high temperatures. Moreover, this concern is even further aggravated by the recent trend toward the use of various stability control agents which provide improved ambient temperature dimensional stability with a wider variety of blowing agents, but which unfortunately also generally lengthen the time required for the olefin polymer foam product to reach its so-called fully cured state.
Additionally, the industry has recently begun using so called "soft" chlorofluorocarbon blowing agents, in particular CFC-142b (1-chloro-1,1-difluoro-ethane), mixtures containing such soft chlorofluoro-carbons or hydrocarbon blowing agents such as isobutane. These compounds have been found to be more readily converted in the environment to decomposition products having reduced deleterious effects. Disadvantageously CFC-142b and isobutane permeate faster through ethylenic polymers than air resulting in foam collapse. To avoid this problem it has become necessary to employ stability control agents to retard the permeation rate of these halocarbon or hydrocarbon blowing agents.
In U.S. Pat. No. 4,368,276 suitable N-substituted fatty acid amides are disclosed for use in polyolefin foams as stability control agents. In certain end uses, such as packaging for sensitive electronic parts, foams must possess electrostatic dissipation properties to prevent an accumulated electrostatic charge on the foam surface from discharging through contact with the electronic part. Although operating effectively for many end uses, it has been discovered that foams prepared utilizing the foregoing N-substituted fatty acid amides do not show measurable improvement in electrostatic dissipation. Moreover, when static control agents such as alkyl amines are added to the formulation the stability control properties of the N-substituted fatty acid amide are adversely affected.
In view of the foregoing, it is an object of this invention to provide olefin polymer foams having, in fresh foam form, good elevated temperature distortion properties as well as to provide olefin polymer compositions which are expandable to form such improved polymer foams Another object of this invention is to provide olefin polymer foams having increased electrostatic dissipation properties. Other objects and advantages of the present invention will be apparent from the description thereof which follows.